A power distribution system typically includes components such as circuit breakers, transformers and power lines. A typical circuit breaker includes sensors for identifying transients in the power line current and controls to trip the breaker, i.e., open the breaker, so as to open a particular branch of the system at the proper instant.
Current sensors conventionally used in circuit breakers and other power system components require complex fabrication techniques and hence are expensive in terms of both material and labor costs. For example, current sensors typically employ a copper wound iron core for sensing current. The core generally must be machined to within very limited tolerances. Materials such as copper and laminated magnetic quality steel, requiring very tight manufacturing tolerances, account for much of the material and labor costs of the sensor.
In reducing the core material size, thus reducing assembly costs of a current sensor, the sensor accuracy preferably is not adversely affected. Accurate current sensing is often required in circuit breaker applications utilizing digital display and metering. Inaccurate current sensing could lead, detrimentally, to unnecessarily cutting-off power to a load and introducing discrepancies in power metering.
In addition to high accuracy and low cost, the size of current sensors is often important. For example, if an existing product must be redesigned in order to accomodate a new current sensor, the cost of adding the new current sensor to the product can be very expensive. Preferably, any new current sensor is sized so that it can be easily installed into the existing product, such as a circuit breaker.
Since many power system components do not include batteries, the current sensor preferably is also self-powered. This generally means that any power required by the current sensing circuit and other electronic components in the breaker unit is provided by the power line being monitored. Avoiding use of an external battery not only avoids adding to the initial cost of the sensor, i.e., the added cost of the battery, but the power system components do not rely on battery power to provide protection if any overcurrent condition exists and the breaker must trip. Use of a self-powered current sensor thus facilitates achievement of high reliability operation.
It would be desirable, therefore, to provide a relatively low cost current sensor which is of reduced fabrication complexity as compared to known current sensors, and is sized so that it can be used in existing circuit breakers. In addition, such low cost current sensor must operate with high accuracy to sense current in a power line, and preferably is self-powered.